1. Field of the Invention
The present invention relates to a liquid crystal display apparatus of the supertwisted nematic type (hereinafter, referred to as "STN-LCD") with a phase difference plate for color compensation, particularly a black-and-white STN-LCD which can be used in an OA (Office Automation) apparatus.
2. Discussion of the Background
In the field of liquid crystal display, the image quality of TN (twisted nematic) type liquid crystal displays has been improved. In a TN type liquid crystal display, nematic liquid crystal molecules have a twisted angle of 90.degree. and a liquid crystal cell is sandwiched between a pair of polarizing sheets with their planes of polarization crossing or being parallel. By interposing a phase retarder between one of the polarizing sheets and the liquid crystal cell, the image quality of the TN type liquid crystal display can be improved as disclosed in Japanese patent publication 61-186937 and 60-26322.
To cope with the demands for an increased display capacity and an enlarged display screen, a super twisted nematic (generally called STN) type liquid crystal display have recently been developed. In an STN type liquid crystal display, the nematic liquid crystal molecules are twisted at an angle of about 180.degree. to 270.degree.. However, the STN type liquid crystal display suffers from "coloring" ascribed to birefringence of the liquid crystal molecules, resulting in a poor black-and-white display. In order to solve this problem, it has been suggested that a liquid crystal cell may be added to the STN type liquid crystal cell as an optical compensator, thereby eliminating the coloring and allowing a good black-and-white display. It has also been suggested that a liquid crystal cell may be combined with a phase retarder as an optical compensator, thereby eliminating the coloring and allowing a good black-and-white display. The phase retarder is a film or sheet having a birefringence character. Since a phase retarder has different refractive indices in two directions crossing at a right angle to each other, light transmitting through the phase retarder produces light rays whose planes of polarization make a phase difference.
In conventional liquid crystal display apparatus, a glass plate is predominantly used as a substrate of a liquid crystal display device. However, a plastic sheet is more preferred since it is light in weight, flexible and hard to break.
A glass plate has in general no optical anisotropy, its retardation value is therefore taken to be zero. However, the retardation value of a plastic sheet is non-zero because of the optically anisotropic nature of the plastic sheet, and usually gives rise to a retardation value in the order of 10 nm.
In an STN type liquid crystal display device, images are displayed by utilizing both rotation of plane of polarization and birefringence, which are induced by a layer of aligned liquid crystal molecules. Because of the polarization and rotation characteristics, the liquid crystal layer has its retardation value.
Accordingly, the STN type liquid crystal display devices with the glass substrate have difficulties such as a decreased brightness and contrast, and undesirable coloring. Furthermore, since a plastic substrate has a non-zero retardation value, as mentioned earlier, its retardation value is superimposed to those of the liquid crystal layer and other optical components, to thereby enhance the above-mentioned difficulties.
In order to obviate these difficulties, therefore, the axes of the optical components in the liquid crystal display device have to be arbitrarily aligned as will be detailed in the present disclosure. The axes of the optical components includes those of lower and upper plastic substrates, alignment treatment, lower and upper polarizing sheets, and phase retarder(s). j